Write down the dev-stack and Playwright traps (sow-nodebb-plugin-support#10)

Assume the dev stack is dirty on arrival and leave it clean; the compiled
templates/client JS need an explicit build, and the build marker lives on the
container filesystem so a plain restart skips it. Also records this forum's
login selectors and the fact that waitForURL('**/') never matches NodeBB's
post-login /?loggedin=true.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -57,6 +57,21 @@ for driving a headless browser on this NixOS box (the Playwright MCP server
 does not work here). Point any browser tooling at `http://localhost:4567` to
 load pages, take screenshots, and compare before/after.
 
+**Assume the stack is dirty when you arrive, and leave it clean.** Reset it
+(`make dev-reset`) before the first boot of a testing session and tear it down
+after every cycle, including a failed one; a stack carrying the last session's
+data is the usual reason a screenshot shows something that no longer exists in
+the code. Full hygiene rules live in `../sow-nodebb/AGENTS.md`. Two things
+that bite here in particular:
+
+- **Templates and client JS are compiled**, so a browser reload alone proves
+  nothing after editing them. `make dev-build` — and note the build marker
+  lives on the container filesystem, so a plain `restart` skips the rebuild.
+- **Playwright login on this forum**: the form is `#username` / `#password`
+  with a `#login` submit button, and a successful login lands on
+  `/?loggedin=true`, which `page.waitForURL('**/')` never matches. Wait on
+  `page.waitForLoadState('networkidle')` instead.
+
 ## Design Context
 
 - `PRODUCT.md`: strategic design context — register, users, purpose,